gn2bp: Fix to use target sources and inputs

`a or b` does not return the union of set a and set b.
Test: ./update_results.sh

Change-Id: I8ba2f2a96e036bab362cbb7015cc6da048e12024
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 42b69c1..2ab52ac 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -959,7 +959,7 @@
   # gn treats inputs and sources for actions equally.
   # soong only supports source files inside srcs, non-source files are added as
   # tool_files dependency.
-  for it in target.sources or target.inputs:
+  for it in target.sources.union(target.inputs):
     if is_supported_source_file(it):
       module.srcs.add(gn_utils.label_to_path(it))
     else: